The present invention relates to what is commonly designated as a five-speed automatic transmission that is particularly adapted for vehicular usage. That is, the transmission provides five forward speeds or gear ratios, and one reverse speed or gear ratio. By way of introduction, and to clarify the discussion which follows, the forward gear ratios are defined as those ratios which causes the vehicle to move forwardly, and, for simplicity, it will be assumed that the configuration of the planetary gear sets is such that in a forward gear ratio the output member rotates in the same direction as the input member. Conversely, the reverse ratio causes the vehicle to move rearwardly, and it will also be assumed that the output member rotates in a direction opposite to that of the input member.
As is also known, an input member is a connecting mechanism (normally a shaft in the transmission) which receives its driving force or torque from the vehicular engine--as through a well known torque converter--and imparts the driving torque to the planetary gear sets which comprise the epicyclic transmission. The output member operatively connects the transmission to the differential from which the drive wheels are rotated.
"Torque transfer device" is another term commonly employed in the description of planetary gear sets. Two forms of torque transfer devices are commonly recognized--viz.: clutch members and brake members. An input clutch member is employed selectively to interconnect two members that are normally relatively rotatable so that they move in unison. A brake member is employed selectively to preclude rotation of a member that is mounted so as be rotatable. Typically, a brake member is presented from the housing in which the transmission is encased, and the brake member thereby serves to "ground" a normally rotatable member to the housing so that member cannot rotate. As will be hereinafter described, in one embodiment of the present invention at least one torque transfer device sometimes operates as a clutch an sometimes as a brake.
A planetary gear set typically consists of a sun gear, which is a small gear located in the center of the planetary set, and a ring gear which, as the outermost member, circumscribes the set and has inwardly-facing teeth. A plurality of planet gears or pinions are interposed between the aforementioned sun and ring gears to be rotatable and/or circumferentially translatable. The planet gears in each planetary gear set are normally supported from a carrier that will rotate in response to translation of the planet gears. Conversely, the carrier may be rotated to effect a desired movement of the planet gears.
Rotation of the sun gear will cause all of the other elements in the planetary set to move, as well, unless one of the other elements is held stationary by a torque transfer device such as a brake member which may be presented from the transmission housing. When a brake member grounds one of the components in a planetary gear set to the housing, the grounded member will be forcibly held in position so that all the other members are movable relative thereto. Selectively grounding the members of a planetary gear set in combination with selectively connecting the input shaft to the desired component of the planetary gear set constitutes the typical means by which different gear ratios are achieved from a planetary gear set--i.e.: different members are selected to be rotated or grounded so the speed and direction of the output can be appropriately changed to accommodate the existing conditions.
There is, however, a limit to the number of variations which can be provided by a single planetary set. As a result, planetary gear sets have been compounded to provide a greater number of individual speeds or gear ratios. When planetary gear sets are compounded, various torque transfer devices are provided to effect selective interconnection between the members of the individual gear sets as well as to ground one or more of the components of the gear sets to the transmission housing. These arrangements, while successful, do tend to increase the length of the housing required to encase not only the gear sets but also the torque transfer devices required to provide the desired number of gear ratios.